Representative input parameters for regulatory exposure modelling can be derived from pesticide concentration measurements in real-world field studies. These can potentially be used in the authorization process of plant protection products. In order to estimate soil degradation rates that solely result from microbial degradation and not from other processes such as leaching, plant uptake, volatilization, and photodegradation, the relevant guidance documents recommend the use of inverse modeling techniques in combination with environmental fate and transport models. With a field leaching study, both degradation and sorption to the soil matrix can be considered. In this study, measured solute concentrations from five different depths down to 1.5 m were used to conduct a parameter estimation to be able to reflect degradation and sorption processes, accordingly. To this end, a coupling of the leaching model PEARL with the universal optimization tool PEST was applied, and values for degradation and sorption parameters of the fungicide fluopicolide and its soil metabolite BAM (2,6-dichlorobenzamide) were inferred.
Compared with results from regular field degradation studies sampling residues in the total soil matrix, soil degradation half-lives were not different (236 d [124-349 d] vs. 158 d [144-173 d] for fluopicolide and 53 d vs. 45 d for BAM), however a kinetic sorption (increase sorption with time) was observed for the parent compound compared to laboratory data. The value for the distribution coefficient (K_f,OC) increased from 248 mg^1-1/nL^1/nkg^-1 (in the lab) to 522 mg^1-1/nL^1/nkg^-1in this field leaching study. This work serves to demonstrate the feasibility of including field leaching studies with measurements at different soil depths in regulatory exposure assessments, as significant degradation and sorption modelling parameters were derived with low uncertainties in the estimated parameter values of ±10%.

Dr. Stephan Sittig

Environmental Fate Modelling Expert, knoell Germany GmbH

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